1. Field of the Invention
This invention relates to the field of polymer chemistry. More particularly, this invention relates to the synthesis of the bismaleimide of dimer diamine. Still more particularly, but without limitation thereto, this invention relates to the synthesis of bismaleimide of dimer diamine using N,N'-dicyclohexylcarbodiimide and 1-hydroxybenzotriazole.
2. Description of the Prior Art
Previous methods for synthesizing the bismaleimide of dimer diamine and other aliphatic bismaleimides, relied upon dehydration of the precursor bismaleamic acid with acetic anhydride, acetic anhydride/sodium acetate or acetic anhydride/triethylamine in an analogous fashion as in the synthesis of aromatic substituted maleimides. However, these reagents continually provide a low yield, 9-41%, of the aliphatic bismaleimide product.
Other general methods which, for example, require heating of the precursor maleamic acids to high temperatures in order to remove water, again result in low yields of maleimide products.
Some work has been done on the synthesis of very low molecular weight, monofunctional aromatic substituted maleimides using N,N'-dicyclohexylcarbodiimide and 1-hydroxybenzotriazole co-reagents. However, no work has been described on the synthesis of polyaliphatic substituted bismaleimides from precursor bismaleamic acids using these co-reagents. A process has also been developed for very low molecular weight monofunctional aliphatic maleimides. This process uses a mineral acid for cyclodehydration followed by elimination of hydrochloric acid to form the maleimide. This process is not feasible with dimer diamine since the polymeric backbone contains unsaturation which is susceptible to addition by hydrochloric acid just as the maleimide double bond is.
The present invention provides a means by which the bismaleimide of dimer diamine can be synthesized in 38-56% yield by a mild method involving cyclodehydration of precursor bismaleamic acid.